1. Field of the Invention
The present invention relates to offset compensation. More particularly, the present invention relates to a burst mode receiving apparatus having an offset compensating function and a data recovery method thereof.
2. Description of the Related Art
In general, unlike an optical transmission system having a point-to-point connection structure, in a passive optical network (PON) having a point-to-multi points connection structure, a number of optical network units (ONUs) each acting as an optical terminal are connected to a central office through fiber optic cables. Presently, only an ONU that has been granted permission by the central office may transmit its own information upstream to the central office in the form of a packet. The upstream-transmitted optical packets reach an optical receiving apparatus of the central office with different optical powers and phases depending on the ONU. The optical receiving apparatus of the central office must temporarily respond to an input optical power varying dynamically in a packet unit, which is called xe2x80x9ca burst mode optical receiving apparatus.xe2x80x9d Such a burst mode optical receiving apparatus is basically provided with a peak detector for detecting a peak value of an optical signal and a comparator for discriminating between xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d using an output of the peak detector as a reference value. A burst mode optical receiving apparatus may be characterized as having a feedback-scheme or a feedforward-scheme depending on the configuration type thereof.
One conventional burst mode optical receiving apparatus adopts a feedback structure in which a peak value of an input optical signal is obtained at an output terminal of a preamplifier and the obtained peak value is input to a sub-input terminal of the preamplifier as a reference voltage. Accordingly, for such a conventional burst mode optical receiving apparatus, even if stability of a circuit is improved, there is a problem in that it is not easy to increase both a gain of the preamplifier and an operative bandwidth using a complementary MOS (CMOS) process. Thus, an expensive bipolar junction transistor (BJT) process must be employed. Also, in the case where a signal of xe2x80x9c0xe2x80x9d in a packet is sustained over a long time period due to a leakage current caused by a base current generated from a transistor fabricated by the BJT process, there is a problem in that a value of voltage charged in a peak capacitor of the peak detector is decreased gradually. Therefore, a reference value may not be set correctly and an external trimming process must be performed for each chip in order to eliminate an offset existing within a limiting amplifier having a very large gain.
Another conventional burst mode optical receiving apparatus includes a preamplifier for amplifying an optically detected electric signal, an automatic bias adjuster for obtaining an intermediate value from a maximum value and a minimum value of the signal, and a limiting section. For such a conventional burst mode optical receiving apparatus of a feedforward-scheme, there is a problem in that, although it may be implemented with a less costly CMOS process, an offset existing within a limiting amplifier allows an output terminal of a limiting amplifier to generate a distorted waveform. Moreover, the feedforward-scheme conventional burst mode optical receiving apparatus encounters additional problems. For example, although the automatic bias adjuster and the limiting section thereof are intended to be connected in multiple stages so that each of the limiter amplifiers amplifies an input optical signal through a series of steps in a linear region to reduce an influence of an offset, the conventional burst mode optical receiving apparatus must use two resistors having the same size to obtain a reference value used to obtain an intermediate value from a maximum value and a minimum value detected by a peak detector. Thus, if the two resistance values are not identical in the manufacture of the conventional burst mode optical receiving apparatus, the reference value cannot be established correctly and it becomes difficult to improve the precision of offset compensation. Moreover, as a gain of an operational amplifier of a feedback-scheme constituting a peak detector decreases, a voltage drop of a peak value of the peak detector is increased and it is difficult to upgrade the peak detector of a feedback-scheme at a higher operational speed.
In an effort to solve the above-described problems, it is a first feature of an embodiment of the present invention to provide a burst mode receiving apparatus that is able to more precisely compensate for an offset without requiring an external tuning and to operate a higher speed.
It is a second feature of an embodiment of the present invention to provide a data recovery method to be implemented in the burst mode receiving apparatus.
To provide the first feature, there is provided a burst mode receiving apparatus having an offset compensating function, including an intermediate value detector to detect an intermediate value between a maximum value and a minimum value of an input signal of an analog form discretely input from an outside source in response to a switching control signal, and output the detected intermediate value; an amplifier to amplify a difference between the input signal and a reference value, and output the amplified result; an offset compensator to generate a compensation signal having a level varied corresponding to the amplified result input from the amplifier and a compensation control signal; a summing portion to add the compensation signal input from the offset compensator and the intermediate value input from the intermediate value detector to output the added result as the reference value to the amplifier; and a controller to generate the switching control signal and the compensation control signal corresponding to a result obtained by analyzing the amplified result input from the amplifier and a reset signal input from the outside source.
To provide the second feature, there is provided a data recovery method to be implemented in a burst mode receiving apparatus, including: (a) compensating for an offset of the burst mode receiving apparatus; (b) preparing for reception of an input signal from an outside source; (c) determining whether the input signal is given from the outside source, and returning to (b) if it is determined that the input signal is not given from the outside source; (d) detecting an intermediate value between a maximum value and a minimum value of the input signal if it is determined that the input signal is given from the outside source; (e) adding a compensation signal and the detected intermediate value to obtain a reference value; (f) amplifying a difference between the input signal and the reference value and determining the amplified result as recovery data; and (g) initializing the maximum value and the minimum value of the input signal.